1. Technical Field
The present invention relates a screen printing apparatus in which each of squeegees can be slid on a mask plate brought into contact with a substrate to scrape paste to thereby fill pattern openings of the mask plate with the paste, and a paste kneading method performed by the screen printing apparatus.
2. Description of the Related Art
A screen printing apparatus in the background art has a configuration in which squeegees can be slid (moved horizontally in abutment) on a mask plate brought into contact with a substrate so that a pattern formed as openings in the mask plate can be transferred to the substrate. Of the paired squeegees provided side by side in the horizontal direction (the sliding direction) above the mask plate, only one is selectively slid in abutment with the mask plate.
Paste is used in such a screen printing apparatus. When the paste is being scraped and rolled on the mask plate, the paste is low in viscosity and high in filling performance into the pattern openings of the mask plate. However, once the rolling of the paste is suspended, the viscosity becomes high and the filling performance into the pattern openings becomes low. Thus, printing accuracy deteriorates. In some screen printing apparatuses, therefore, a paste kneading operation is executed when a screen printing operation has been suspended continuously for a long time (e.g. for 30 minutes). In the paste kneading operation, a pair of squeegees are slid simultaneously within a predetermined area on the mask plate (normally an area on the mask plate in contact with clampers clamping the substrate) to roll the paste (e.g. JP-A-9-193343).
However, in the screen printing apparatus according to the background art, the two squeegees simultaneously move back and forth in abutment with the mask plate during the paste kneading operation. Accordingly, after scraping the paste within the kneading area, each of the squeegees moves while spreading the paste in an opposite direction to the direction in which the squeegee has scraped the paste till then. Therefore, there is a problem that printing failure may occur due to air mixed into the paste in a screen printing operation performed afterward or in a paste kneading operation performed successively.